Season 17, Episode 15: Why is it So Hard to Detect Gravitons?

Hey StarTalkians! Season 17, episode 15 was another grab-bag Cosmic Queries session with Neil and Chuck, and one question touched on the ongoing search for the “graviton”:

Cosmic Queries – Gravitons & Hyperspeed - StarTalk Radio

(from 16:50)

As Neil explains, they’re trying. His answer touched on some key points, but as usual for Cosmic Queries sessions, they weren’t able to go into much detail. So it’s natural to wonder: why is it so hard to detect gravitons?

Dyson’s Declaration – Setting the Challenge

Physicist Freeman Dyson made a bold suggestion in 2004 that sent waves through the physics community: that it would be impossible to detect a single graviton.

The problem is that the theory strongly suggests they exist. Electromagnetism is mediated through photons (a boson), the strong and weak nuclear forces are due to W and Z bosons and so theorists expect a “boson” for gravity too: the graviton.

Why It’s So Hard to Detect Gravitons

Shortly after Dyson’s famous comment, a paper took up the challenge. Using some generous assumptions, they looked at whether it really wasn’t physically possible to detect gravitons.

These assumptions give you an idea of how hard this would be: they assume a detector as massive as Jupiter, and that you could put such a detector really close to a source of gravitons, like a white dwarf.

Even in this bizarre, sci-fi situation, they found that gravitons from most sources would be undetectable. In fact, only “braking radiation” (aka “bremsstrahlung”) could produce detectable gravitons. Other bold ideas – including from the decay of primordial black holes – stood no chance.

Among other issues, neutrinos would be produced 10^33 times more than gravitons. There are about 10^22 grains of sand on Earth. This is like every grain of sand on more than a hundred billion Earths for just a single graviton.

You could shield against neutrinos, but the paper’s math reveals an issue. The shield would have to be so big it would become a black hole.

Modern Approaches are More Promising

Surprisingly, though, modern estimates are more optimistic. Physicists have proposed “catching” a gravitational wave using a quantum state, in a setup a little like the photo-electric effect (see attached figure). It’s even possible that they’ll be able to do this with next-generation gravitational wave detectors.